In recent years, with an explosive increase in demand for communications using the Internet, there has been a growing need for increasing the transmission capacity of an optical transmission system for a truck line. Presently, the optical transmission system for the truck line uses a wavelength band of about 1,530 to 1,560 nm (C band), which is an amplified wavelength range of an Er-doped fiber amplifier, as well as a wavelength band of about 1,570 to 1,600 nm (L band). Moreover, to increase the transmission capacity, the use of a different wavelength band has been proposed. Thus, systems using a new wavelength band around 1,470 nm (S band: about 1,470 to 1,530 nm) are researched and developed (refer to T. Sakamoto et al., OFC2000, PD4).
The optical transmission system comprises a large number of optical passive parts such as filters and couplers. During a process of manufacturing these optical passive parts, the parts are inspected for insertion losses or cross talk. For such inspections, a light source is used which can be used over a relatively wide wavelength range in order to make the operation simpler and more efficient. For example, an ASE light source using Er-doped optical fibers is used for optical passive parts used in the above C or L band.
The ASE (Amplified Spontaneous Emission) light source provides as outputs spontaneous emission light of a wide band from optical fibers in which rare earth is doped. The ASE light source using Er-doped light fibers provides high outputs in the C and L bands. It thus offers a power density of about −20 dB/nm, which is required to measure cross talk in a narrow-band filter such as an AWG (Arrayed Waveguide Grating).
FIG. 1 shows the spectrum of a conventional ASE light source. A practical ASE light sources now have a power density of at least −20 dBm/nm in two wavelength bands of 1,440 to 1,490 nm and 1,525 to 1,605 nm. However, in a wavelength band of 1,490 to 1,525 nm, the light source offers a reduced power density and cannot be used for optical measurements. The wavelength of 1,490 to 1,525 nm is expected to be utilized as the WDM transmission technology advances. Accordingly, optical parts required to construct a system must be developed. Furthermore, alight source is required which is used to evaluate the characteristics of the optical parts.
Such a light source is not limited to the ASE type but a wavelength-variable laser may be used. However, the wavelength-variable laser cannot be economically constructed. On the other hand, fiber lasers are economical and easy to manufacture. They thus offer significant advantages to the optical part industry. Moreover, a rare earth-doped optical amplifier has not been put to practical use yet which offers a sufficiently high gain and a sufficiently small noise factor over the above described wide wavelength range. Realization of this amplifier leads to the advancement of the WDM transmission technology.
ASE light sources are known which use Tm-doped optical fibers in the new wavelength band around 1,470 nm. However, these light sources offer only a low output power density and a narrow wavelength range; they do not offer a power density of at least −20 dBm/nm, which is required for measurements in the S, C, and L bands.
It is an object of the present invention to provide an ASE light source that can provide high outputs even in the wavelength band of 1,490 to 1,525 nm, an ASE light source that can continuously cover the S, C, and L bands, an optical amplifier that can operate in the S and C bands, and a laser oscillator that can switch the wavelength between the S band and the C band.